Add remove-em-dash hook

New fixer hook that replaces UTF-8 em-dashes (U+2014) with a plain
hyphen (-), modeled on the trailing-whitespace hook.

- pre_commit_hooks/remove_em_dash.py: the fixer (binary-safe, UTF-8 only)
- tests/remove_em_dash_test.py: full coverage of fix and no-op cases
- registered in setup.cfg, .pre-commit-hooks.yaml, and README.md

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/remove_em_dash_test.py

@@ -0,0 +1,36 @@
+from __future__ import annotations
+
+import pytest
+
+from pre_commit_hooks.remove_em_dash import main
+
+
+@pytest.mark.parametrize(
+    ('text', 'expected'),
+    (
+        ('foo\N{EM DASH}bar\n', b'foo-bar\n'),
+        ('foo \N{EM DASH} bar\n', b'foo - bar\n'),
+        ('a\N{EM DASH}b\N{EM DASH}c\n', b'a-b-c\n'),
+        ('x\N{EM DASH}y\r\nz\r\n', b'x-y\r\nz\r\n'),
+    ),
+)
+def test_fixes_em_dash(text, expected, tmpdir):
+    path = tmpdir.join('file.txt')
+    path.write_binary(text.encode())
+    assert main((str(path),)) == 1
+    assert path.read_binary() == expected
+
+
+@pytest.mark.parametrize(
+    'contents',
+    (
+        pytest.param(b'foo-bar\n', id='plain-hyphen'),
+        pytest.param(b'no em dashes here\n', id='no-dash'),
+        pytest.param(b'<a>\x97</a>\n', id='windows-1252-em-dash'),
+    ),
+)
+def test_noop_without_utf8_em_dash(contents, tmpdir):
+    path = tmpdir.join('file.txt')
+    path.write_binary(contents)
+    assert main((str(path),)) == 0
+    assert path.read_binary() == contents